The following patents, patent publications and publications are believed to represent the current state of the art:
U.S. Pat. Nos. 4,135,183; 5,884,271; 7,268,687; 7,591,415; 7,806,333; and 7,834,816;
U.S. Published Patent Application Nos.: 2007/009624 and 2010/0213261;
Japanese Patent JP3832363;
Bernhard et al., RFID in Metallic Environment, http://www.rfid-systech.eu/20070613_3A_1010_Bernhard_RFIDInMetallicEnvironment.PDF; and
AAN PT9S Long Range Antenna for Cattle ID, http://www.trovan.com/products/FDXB/FDXBfixed/FDXBfixed.html, commercially available from NSG Digital Systems of Kedah, Malaysia.